1 Distributed volcanic fields of the southwestern U.S.

This session invites submissions from various disciplines (e.g., petrology, tectonics, geophysics, geochronology) that explore distributed volcanism in the southwestern U.S. Topics of interest include spatio-temporal trends, magma compositions, and mantle and/or crustal controls on volcanism.

Monday, 18 May 2026
8:00 AM - 12:00 PM
Alvarado A

Melt and Fluid Inclusion Insights into Magma Compositions and the Plumbing System Beneath Merriam Crater Cinder Cone, San Francisco Volcanic Field, Arizona

Paper number: 1-12
Program: Technical Sessions
Time: 11:35 AM – 11:55 AM

Authors

Abstract

The San Francisco Volcanic Field (SFVF) in northern Arizona contains over 600 cinder cones and maar craters; the youngest, Sunset Crater, erupted ca. 1085 CE. Here we use olivine-hosted melt and fluid inclusions from Merriam Crater, a large-volume cinder cone and the second-youngest eruption (~19 ka) in the SFVF, to investigate magma compositions and the plumbing system that fed this eruption.

Merriam Crater melt inclusions were analyzed by FTIR (H2O and CO2) and electron microprobe (major and minor elements, S, Cl). The melt inclusion glasses are alkali basalts with 5.5–9.5 wt% MgO and are trapped in Fo80-87 olivine. Melt inclusion compositions have been corrected for post-entrapment crystallization and the presence of vapor bubbles using MIMiC1; corrected compositions are variably degassed, with 0.5–1.6 wt% H2O and 400–5000 ppm CO2, corresponding to entrapment pressures of ~100–500 MPa. These results are similar to entrapment pressures found for Sunset Crater olivine-hosted melt inclusions2. Olivine from Merriam Crater also contain fluid inclusions; analyses by Raman spectroscopy indicate fluid inclusion densities of ~0.6–0.95 g/cm3, corresponding to trapping pressures of ~300–750 MPa. Although there is some overlap with the melt inclusion-based pressure estimates, fluid inclusions pressures are overall higher, indicating either that fluid inclusions were preferentially trapped in deeper crystallizing olivine, or, more likely, that reconstructed melt inclusion CO2 is underestimated. Altogether, our results demonstrate that the Merriam Crater magma likely crystallized olivine during ascent between ~26–10 km depth, roughly the lower to middle crust, with little or no shallower storage and crystallization. These depths are more variable and extend to higher values than those estimated for the Sunset Crater magma2 (~12–17 km). Melt inclusion sulfur concentrations from Merriam Crater (400–1980 ppm) are more variable than Sunset Crater (1600–2200 ppm2) consistent with syn-ascent crystallization and thus trapping of a progressively degassing magma.

Overall, like Sunset Crater2, we find that high-CO2, sulfur-rich magmas erupted at Merriam Crater, and suggest such gas-rich magmas could be common in SFVF eruptions. Our work also indicates that magma crystallization may occur dominantly in the mid-lower crust during ascent, rather than during shallow storage, complicating detection of future magma intrusions into the crust.

1.        Rasmussen, DJ,  et al., Am Min, 2020.

2.        Allison, CM, et al., Nat Comm, 2021

Geological Society of America Abstracts with Programs. Vol. 58, No. 4

doi: 10.1130/abs/2026RM-410670

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